1. Field of the Invention
The present invention relates to a pattern verification-test method, an optical image intensity distribution acquisition method, and a computer program product.
2. Description of the Related Art
In a recent semiconductor manufacturing process, a problem in a photolithography process has become more significant. This is because as the size of a semiconductor device become smaller, finer photolithography process is required. The design rule of a semiconductor device has become as small as 45 nanometers, and the pattern dimensional accuracy that is required to be controlled is 3 nanometers or smaller. Furthermore, because optical proximity correction (OPC) is performed on a mask pattern, the mask pattern ends up having an extremely complicated shape. Therefore, a conventionally performed one-dimensional uniformity test such as measurement of a pattern line width or measurement of a hole diameter is insufficient, and two-dimensional control is demanded.
Dimensional management of a mask pattern based on an image of a mask pattern acquired with a scanning electron microscope (SEM) is one example of the two-dimensional control methods. In this method, contour of a mask pattern is extracted from the acquired image of the mask pattern, lithography simulation (transferability evaluation) is performed on the contour, and it is judged whether desired lithography latitude can be obtained. Thus, the mask pattern is tested whether it has a desired dimension. An advantage of the dimensional control method using a SEM is that the facon of the mask pattern can be determined under a condition that the mask pattern is exposed on a wafer (the condition extremely similar to that of actual use), and that necessary and sufficient management can be performed without being excessively strict or lenient (M. Kariya et al., “Reticle SEM Specfications Required for Lithography Simulation”, Proceedings of SPIE Vol. 5853 550-555, 2005).
However, the accuracy of the conventional technique cannot meet the demand of the recent strict dimensional control of a mask pattern. This is because the conventional technique has ignored the influence of irregularity of illumination from a wafer exposure apparatus on wafer exposure. For example, when irradiating a transmissive mask with exposure light, the shape of exposure light illuminating the mask (the illumination shape) should be uniform at any position in the mask surface. However, in fact, in the case of an orbicular zone illumination for example, although the illumination shape is an orbicular zone close to a substantially perfect circle near the center of the mask, the illumination shape is distorted to be an elliptical shape at a position closer to the periphery of the mask. Accordingly, the results (pattern dimensions) of performing lithography simulation with an ideal illumination, and of exposing the mask pattern on a wafer actually do not match with each other in some cases.
EUV lithography using a reflective mask has a problem similar to that of the lithography using a transmissive mask. In a case of the EUV lithography, an incidence direction of EUV light incident on a mask is different depending on a position of the mask. However, in the conventional technique, the same optical parameters are used in a mask surface for lithography simulation. Thus, an error occurs in lithography simulation because although the incidence direction (the illumination shape) of EUV light is different in a mask surface, the position in the mask surface from which a SEM image is acquired is ignored. Accordingly, the pattern dimension of a mask pattern of a transmissive mask and a reflective mask cannot be tested accurately.
Not only in testing a mask pattern formed on a photomask, but also in performing mask data process such as optical proximity correction in designing of a mask pattern, the influence of irregularity of an illumination shape has not been considered. Accordingly, an accurate optical image intensity distribution cannot be obtained when lithography simulation is performed by using a mask pattern (data), and appropriate verification and designing of a mask pattern have not been possible.